1. Field of the Invention
The present invention relates to a packet transmission node device which functions as a router or a host for transmitting and receiving packets by being connected to a virtual connection oriented network, and a packet transfer scheme and a control information transfer scheme used in a packet transmission node device.
2. Description of the Background Art
A virtual connection oriented network is a network based on techniques such as ATM (Asynchronous Transfer Mode), frame relay, fiber channel, HIPPI (High Performance Parallel Interface), etc.
On such a network, it is possible to realize a multicast in which a packet sent by a sender to a destination address for multicast (a multicast address) in a network layer is distributed to a plurality of receivers by the network. In the following, for the sake of simplicity, a case of an IP (Internet Protocol) multicast communication on an ATM network which uses IP as the network layer will be described.
FIG. 1 shows a conventional scheme for transferring a packet destined to a multicast address G in an IP multicast communication system. In FIG. 1, H0 to H9 are hosts, where H0 is a sender, and H1, H2, H4, H5, H6, H7 and H8 are receivers. R is a router for transferring a packet, which is connected with the hosts H0 to H9 through ATM connections.
When the network layer is IP, each node (host or router) belongs to a logical group called IP subnet. The communication within the same IP subnet is realized by the direct transfer without passing through a router, while the communication between different IP subnets is realized by utilizing a packet transfer at a router. In FIG. 1, H0, H8 and R belong to an IP subnet A, H1, H2, H3 and R belong to an IP subnet B, H4, H5, H6, H7 and R belong to an IP subnet C, and H9 and R belong to an IP subnet D.
The direct transfer within the same IP subnet can be realized by using either a point-to-multipoint ATM connection or a multicast server (MCS) for carrying out a transfer by duplicating a packet.
In such an IP multicast communication system, a conventional procedure for transferring a packet sent to the multicast address G from a host H0 is as follows.
First, the host H0 transfers a packet destined to G to the host H8 and the router R by using a point-to-multipoint ATM connection. The host H8 then receives this packet and processes the received packet suitably, while the router R transfers this packet to the other subnets (subnets B and C) which have hosts participating in this multicast address G.
In the subnet B, the packet is transferred from the router R to the multicast server MCS by using a point-to-point ATM connection, and the multicast server MCS transfers this packet to the hosts H1 and H2 by using a point-to-multipoint ATM connection. On the other hand, in the subnet C, the packet is transferred from the router R to the hosts H4, H5, H6 and H7 by using a point-to-multipoint ATM connection. The subnet D has no host which is participating in this multicast address G, so that the packet is not transferred to the subnet D.
In this manner, a conventional multicast communication scheme uses one point-to-multipoint connection or one point-to-point connection to a multicast server in transferring the packet between nodes, so that only one and the same quality of service can be provided with respect to one multicast address, and it has been impossible to realize a multicast in which different qualities of service can be provided with respect to different receivers participating in one and the same multicast address.
Now, at a time of transferring a packet to a next hop node from a packet transmission node such as a router or a host connected to the virtual connection oriented network, a virtual connection to that next hop node is set up according to a prescribed connection set up protocol, and a packet transfer is carried out by using a set up virtual connection.
The packets exchanged between transmitting and receiving terminal nodes include actual data between terminal applications as well as various types of control messages, such as a message for a resource reservation for the purpose of requesting a desired communication quality.
In a currently available router connected to the virtual connection oriented network, the identical transfer processing is applied to all the packets which have the identical destination node address, regardless of whether each packet is carrying the user data or the control message. Namely, in a case of the transfer to the virtual connection oriented network, a routing table is referred according to the destination node address of each packet, and then each packet is transmitted to the identical virtual connection according to the routing table, regardless of whether each packet is carrying the user data or the control message.
However, when the control information and the actual communication information are transferred through the same virtual connection, the following problems arise.
(1) In a case of adopting the expansion of the router architecture as described in the Internet Draft "draft-katsube-router-atm-overview-00.txt" (which realizes a faster transfer processing by directly connecting an input virtual connection and an output virtual connection inside a router, without carrying out a network layer processing), it becomes impossible to recognize the control message at a router.
In this case, there is a need to provide a mechanism for extracting only those frames (or cells) received from one virtual connection which correspond to the control message packets, and carrying out an appropriate processing to the extracted control message packets, so as to be able to transfer the other user data frames (or cells) without carrying out the packet level processing. However, an incorporation of such a mechanism causes an increase of a cost for the packet transmission node.
(2) When a problem occurs in the virtual connection which is transferring the user data, the transfer of both the user data and the control message will be stopped, so that there is a possibility for a case in which a handling such as a substitute route selection required at a time of the problem in the virtual connection cannot be carried out properly.
(3) It is impossible to control a plurality of related virtual connections by using a single virtual connection.